1. Field of the Invention
This invention relates to a breathing gases supply and gases humidification apparatus and a method of controlling such apparatus. Uses for the breathing assistance apparatus of the present invention are the supply of gases for any medical condition or treatment that results in the drying of the upper airways, and/or salivary glands, such as head or neck radiotherapy or long term oxygen therapy. Further uses are for medical conditions that result in impaired mucociliary clearance systems such as Chronic Obstructive Pulmonary Disease (COPD) or bronchiectasis or in the supply of Continuous Positive Airway Pressure (CPAP) to treat Obstructive Sleep Apnoea (OSA) or other respiratory conditions.
2. Summary of the Prior Art
A number of methods are known in the art for assisting a patient's breathing. Continuous Positive Airway Pressure (CPAP) involves the administration of air under pressure to a patient, usually by a nasal mask. It is used in the treatment of snoring and Obstructive Sleep Apnoea (OSA), a condition characterised by repetitive collapse of the upper airway during inspiration. Positive pressure splints the upper airway open, preventing its collapse. Treatment of OSA with nasal CPAP has proven to be both effective and safe, but CPAP is difficult to use and the majority of patients experience significant side effects, particularly in the early stages of treatment.
CPAP is also commonly used for patients with a variety of respiratory illnesses, including COPD.
Upper airway symptoms adversely affect treatment with CPAP. Mucosal drying is uncomfortable and may awaken patients during the night. Rebound nasal congestion commonly occurs during the following day, simulating a viral infection. If untreated, upper airway symptoms adversely affect rates of CPAP use.
Increases in nasal resistance may affect the level of CPAP treatment delivered to the pharynx, and reduce the effectiveness of treatment. An individual pressure is determined for each patient using CPAP and this pressure is set at the patient interface. Changes in nasal resistance affect pressure delivered to the pharynx and if the changes are of sufficient magnitude there may be recurrence of snoring or airway collapse or reduce the level of pressure applied to the lungs. Such symptoms can also occur in a hospital environment where a patient is on a respirator. Typically in such situations the patient is intubated. Therefore the throat tissue may become irritated and inflamed causing both distress to the patient and possible further respiratory problems.
A number of methods may be employed to treat such upper airway symptoms, including pharmacological agents to reduce nasal disease, or heating the bedroom. One most commonly employed method is humidification of the inspired air using an in line humidifier. Two types of humidifier are currently used. Cold pass-over humidifiers rely on humidifying the air through exposure to a large surface area of water. While they are cheap, the humidity output is low at high flows, typically 2 to 4 mg/L absolute humidity at flows above 25 L/min. The output is insufficient to prevent mucosal drying. Heated water bath humidifiers are more efficient, and produce high levels of humidity even at high flow rates. They are effective at preventing upper airway mucosal drying, prevent increases in nasal resistance, and are the most reliable means of treating upper airway symptoms.
Oxygen is the most common drug prescribed to hospitalized patients. The delivery of oxygen via nasal cannula or facemask is of benefit to a patient complaining of breathlessness. By increasing the fraction of inspired oxygen, oxygen therapy reduces the effort to breathe and can correct resulting hypoxia (a low level of oxygen in the tissues).
The duration of the therapy depends on the underlying illness. For example, postoperative patients may only receive oxygen while recovering from surgery while patients with COPD require oxygen 16 to 18 hours per day.
Currently greater than 16 million adults are afflicted with COPD, an umbrella term which describes a group of lung diseases characterized by irreversible airflow limitation that is associated mainly with emphysema and chronic bronchitis, most commonly caused by smoking over several decades. When airway limitation is moderately advanced, it manifests as perpetual breathlessness, without physical exertion. Situations such as a tracheobronchial infection, heart failure and also environmental exposure can incite an exacerbation of COPD that requires hospitalization until the acute breathlessness is under control. During an acute exacerbation of COPD, the patient experiences an increase in difficulty of breathing (dyspnea), hypoxia, and increase in sputum volume and purulence and increased coughing.
Oxygen therapy provides enormous benefit to patients with an acute exacerbation of COPD who are hypoxic, by decreasing the risk of vital organ failure and reducing dyspnea. The major complication associated with oxygen therapy is hypercarpnia (an elevation in blood carbon dioxide levels) and subsequent respiratory failure. Therefore, the dose of oxygen administered can be critical and must be precisely known.
To accurately control the oxygen dose given to a patient, the oxygen-enriched gas must exceed the patient's peak inspiratory flow to prevent the entrainment of room air and dilution of the oxygen. To achieve this, flows of greater than 20 L/min are common. Such flows of dry gases cause dehydration and inflammation of the nasal passages and airways if delivered by nasal cannula. To avoid this occurrence, a heated humidifier is used.
The majority of systems that are used for oxygen therapy or merely delivery of gases to a patient consists of a gases supply, a humidifier and conduit. Interfaces include face masks, oral mouthpieces, tracheostomy inlets and nasal cannula, the latter having the advantage of being more comfortable and acceptable than a facemask.
It is usual for the gases supply to provide a constant, prescribed level of gases flow to the humidifier. The humidifier and conduit can then heat and humidify the gases to a set temperature and humidity before delivery to the patient. It is important to note that the warm-up time required from start-up for the gases to reach optimal temperature and humidity increases with higher flow rates. The operating instructions of such a system commonly instruct the user not to connect the system to the patient until the humidifier has completed the warm-up period. Thereafter, patients receive up to 40 L/min of near body temperature saturated gases. Patients often feel overwhelmed by sudden delivery of high flow at this time.
A group of patients who would benefit from humidification therapy are patients who have mucociliary clearance deficiencies. These patients often have purulent mucus and are susceptible to infections from pathogens. Heated humidified air with an abundance of water particles is an ideal medium to harbour disease carrying pathogens. Consequently, considerable design expertise has been required to provide the market with active pass-over humidifiers that deliver water molecules, in gas phase only, so that it is not possible for disease pathogens to be carried in air to the patient. Water that condenses on the inner surfaces of the breathing circuit or conduit at the end of a treatment session may harbour pathogens that would be delivered to the patient next time they use the device. This is particularly the case with therapies for COPD patients that are receiving body temperature fully saturated air.
The hygiene of a breathing circuit (the tubing supplying humidified gases to a patient) is particularly important when the humidification therapy is used for the treatment of respiratory diseases. Any moisture remaining in the breathing circuit at the end of a treatment may harbour pathogens exhaled or expelled (as mucus may be expelled into the circuit or patient interface) by the patient. This moisture provides a means to transport the pathogens in the tubing providing a source of further infection for the patient when the tubing is next used. Often with such treatment the tubing is not cleaned daily and therefore must be thoroughly dried at the end of treatment. This cleaning is time consuming and will not always be carried out to precise instructions.